For works of mixed authorship in which different functions are carried out by different people, a judgment must be made as to whether one function and the person or corporate body which carried it out are so predominant as to render that person or body significant for identification of the work. For example, for purely musical performances, consider the composer of the music to be the principal author unless improvisation or adaptation is so significant as to create a new related musical work by the improviser or adapter. (However, most dramatico-musical works, such as operas, are works of mixed authorship. These are not purely musical works.)